The present invention relates to transducer arrays for generating and focusing ultrasound energy distributions.
There has recently been much interest in developing minimally invasive therapeutic ultrasound techniques for surgery (eg. tissue ablation) or short duration high-intensity hyperthermia, since these may offer potential benefits compared with conventional approaches in terms of reduced morbidity, increased patient acceptability and reduced in-patient time.
Much of the work reported in the field of ultrasound induced ablation has involved the use of single, or a few, piezoceramic transducers with spherical curved surfaces. Examples are described in: C R Hill et al, xe2x80x9cReview article: High intensity focused ultrasoundxe2x80x94potential for cancer treatmentxe2x80x9d, Br J Radiology, vol. 68, pp. 1296-1303; I H Rivens et al, xe2x80x9cDesign of focused ultrasound surgery transducersxe2x80x9d, IEEE Trans. Ultras. Ferroelec. Freq. Ctrl., vol. 43, pp1023-1031; S Madersbacher et al, xe2x80x9cTissue ablation in benign prostatic hyperplasia with high-intensity ultrasoundxe2x80x9d, Eur. Urol., vol. 23 (suppl. 1), pp39-43; and A Gelet et al, xe2x80x9cHigh-intensity focused ultrasound experimentation on human benign prostatic hypertrophyxe2x80x9d, Eur. Urol., vol. 23 (suppl. 1), pp. 44-47, 1993.
However, a significant disadvantage of using a single focused transducer is its fixed focal length. Since the volume of the ultrasound focus is usually smaller than the volume of tissue to be ablated, a means for mechanically translating the transducer must be incorporated. Since it is possible to ablate approximately 2 cm3 of tissue per hour using a single focused transducer, treatment of a modest tissue volume (say 8 cm3) may require multiple sessions totalling 4 hours. Thus, whilst adequate for experimental studies and preliminary clinical testing, mechanical scanning of a single transducer presents a serious practical limitation that may prevent it becoming a routine clinical procedure.
It is possible to reduce such problems by the use of phased arrays in which a plurality of transducer elements are mounted on a substrate surface and collectively provide a focused beam of ultrasound. Such arrays are generally described in: C A Cain et al, xe2x80x9cConcentric-ring and sector-vortex phased-array applicators for ultrasound hyperthermiaxe2x80x9d, IEEE Trans. Microwave Theory Tech., vol. MTT-34, pp542-551; E S Ebbini et al, xe2x80x9cA spherical-section ultrasound phased-array applicator for deep localized hyperthermiaxe2x80x9d, IEEE Trans. Biomed. Eng., vol. 38, pp634-643; S Umemura et al, xe2x80x9cAcoustical evaluation of a prototype sector-vortex phased-array applicatorxe2x80x9d, IEEE Trans. Ultrason. Ferroelec. Freq. Contr., vol. 39, pp32-38; S A Goss et al, xe2x80x9cSparse random ultrasound phased array for focal surgeryxe2x80x9d, IEEE Trans. Ultras. Ferroelec. Freq. Ctrl., vol. 43, pp. 1111-1121, 1996; E B Hutchinson et al, xe2x80x9cDesign and optimization of an aperiodic ultrasound phased array for intracavitary prostate thermal therapiesxe2x80x9d, Med. Phys., vol. 23, pp767-776; E B Hutchinson et al, xe2x80x9cIntracavitary ultrasound phased array for non-invasive prostate surgery,xe2x80x9d IEEE Trans. Ultras. Ferroelec. Freq. Ctrl., vol. 43, pp 1032-1042; H Wan et al, xe2x80x9cUltrasound surgery: comparison of strategies using phased array systemsxe2x80x9d, IEEE Trans. Ultras. Ferroelec. Freq. Ctrl., vol. 43, pp. 1085-1097; K Hynynen et al, xe2x80x9cFeasibility of using ultrasound phased arrays for MRI monitored non-invasive surgeryxe2x80x9d, IEEE Trans. Ultras. Ferroelec. Freq. Ctrl., vol. 43, pp1043-1053; L R Gavrilov et al, xe2x80x9cA method of reducing grating lobes associated with an ultrasound linear phased array intended for transrectal thermotherapyxe2x80x9d, IEEE Trans. Ultras. Ferroelec. Freq. CtrI, vol. 44, pp1010-1017.
Such phased arrays offer electronically controlled dynamic focusing and the ability to vary and control precisely the range and location of the focus during treatment without moving the array. The use of phased arrays offers means of not only rapidly scanning the ultrasound focus but also of synthesizing fields with multiple simultaneous foci. Their use is expected to reduce the time taken to deliver ablative therapy. Several references above propose the use of phased arrays in which elements are placed on a spherical shell, thereby combining electronic and geometric focusing.
A significant disadvantage of known phased arrays is the unwanted presence of grating lobes and other unpredictable secondary intensity maxima which can potentially lead to injuries to a patient undergoing surgery, where excessive energy is deposited into tissue outside the focal region. Another disadvantage, particularly for extracorporeal, two-dimensional arrays, is complexity and potentially relatively high cost.
The need to reduce grating lobes is common to all therapeutic arrays reported to date, and several techniques including apodization, broad banding and the use of subsets of elements have been investigated. The use of a random distribution of different sized elements in a linear phased array has been investigated. Grating lobe levels associated with an array with an aperiodic distribution of elements are approximately 30%-45% less than those associated with periodic centre-to-centre spacing.
It is an object of the present invention to provide an ultrasound transducer array which significantly improves the ability to control precisely the range and location of the focus of the ultrasound energy.
It is a further object of the present invention to provide an ultrasound transducer array which significantly improves the ability to vary the range and location of the focus of the ultrasound energy during use without moving the array.
According to one aspect, the present invention provides an array of ultrasound transducer elements dispersed across a substrate surface, for focusing ultrasound energy over a predetermined focal volume, the elements being dispersed over the substrate surface in a quasi-random distribution, the total radiating area of the elements occupying between approximately 40 and 70% of the total array area.
Preferably the frequency of operation of the ultrasound elements and the average diameter of the elements are related according to the expression:
d=Axc3x97c/f
where d=average diameter of the elements, c=velocity of sound in the medium to be irradiated with the ultrasound energy, lying in the range 1400 to 1600 msxe2x88x921, f=frequency of ultrasound energy and A is a value lying in the range approximately 0.5 to 5.
Preferably, the transducer array substrate surface is a curved shell in which the transducer elements are adapted to radiate from the concave surface of the shell and in which the array has an average diameter, D, which is greater than or equal to 0.7R where R is the radius of curvature of the shell and in which the value of R lies in the range 70 to 200 mm.
Preferably the frequency f lies in the range 0.5 to 3 MHz, and more preferably lies in the range 0.5 to 2 MHz, or 1 to 2 MHz.
According to another aspect, the present invention provides a method of providing localised ultrasonic heating to tissue within a body, using a phased array of ultrasound transducer elements which are dispersed across a substrate surface to focus ultrasound energy over a predetermined focal volume, comprising the steps of:
dispersing the elements over the substrate surface in a quasi-random distribution, with the total radiating area of the elements occupying between approximately 40 and 70% of the total array area,
selecting a frequency of operation of the ultrasound elements and the average diameter of the elements related according to the expression:
d=Axc3x97c/f
where d=average diameter of the elements, c=velocity of sound in the tissue medium being irradiated with the ultrasound energy, f=frequency of ultrasound energy and A is a value lying in the range approximately 0.5 to 5.